1. Field of the Invention
This invention relates, generally, to cooling temperature superconducting power devices. More specifically, it relates to a cryogenic cooling system for superconductor (e.g., high temperature superconductor (HTS)) devices using a gaseous cryogen, for example helium, hydrogen, neon, other suitable pure element, or a gaseous mixture.
2. Brief Description of the Related Art
Power cables have terminations on each end to maintain dielectric integrity. Terminations interconnect the power cable with its high electric field to air-insulated components with lower electric fields and changing ambient conditions. In the case of a superconducting power cable, the terminations act as an interface between the cable and the grid and manage the thermal gradient from the cryogenic temperature components to the ambient temperature components. The terminations additionally need to link the cryogenic environment in the cable with the ambient temperature environment of the non-superconducting elements of the power system, such as copper cables, power transformers, circuit breakers, instrumentation transformers, and disconnect switches.
Superconducting power devices, such as cables, fault current limiters, or transfers, need feedthroughs that connect them with other elements of the power system that stay at ambient temperature. The higher temperatures of these components cause substantial heat influx into the terminations and consequently into the superconducting cable if no countermeasure is installed. It is very important to minimize the heat influx to maintain the operating temperature of the superconducting cable well below critical levels, as well as to minimize the installed cryogenic capacity and operating costs of the superconducting cable system. It must be ensured that the superconducting device remains at the designed operating cryogenic temperature in order to remain effective before, during, and after use. In many cases, when a superconducting device has a temperature of even one (1) Kelvin above the designed operating cryogenic temperature range, the device can quench and would subsequently fail to allow current to flow free of resistance, thus rendering ineffective the superconducting properties of the device.
The standard method of cooling for the cryogenic temperatures required for high temperature superconducting power devices is to use liquid nitrogen in its liquid temperature range of 63-77 K at standard pressure. This is common in High Temperature Superconducting (HTS) power systems [1]-[3]. Typically, the copper conductor and superconductor are positioned directly in the liquid coolant. In situations of forced cooling, fans are utilized, or for cryogenics, in particular, conduction cooling is performed using cryocoolers. However, these methodologies have several drawbacks for certain applications, for example shipboard power systems. One of the greatest disadvantages of this conventional technology is the use of liquid cryogens. In certain environments, liquid cryogens can pose potential unacceptable, unsafe asphyxiation hazards, as well as high pressure and explosion hazards associated with phase change. These disadvantages are described in [18]. Liquid cryogens also limit the temperature range of operation to the temperature at which the cryogen remains liquid.
There have been various attempts to improve upon previous methods of cooling superconducting devices. U.S. Pat. No. 6,854,276 B1 to Yuan et al. (“Yuan”) discusses a method and apparatus of cryogenic cooling for high temperature superconductor devices. Yuan pressurizes liquid cryogen to above atmospheric pressure to improve its dielectric strength, while sub-cooling the liquid cryogen to below its saturation temperature. This method allows for cooling of high-voltage HTS materials without degrading the dielectric strength of liquid nitrogen. Despite the proposed advantages associated with the Yuan system, there are several disadvantages. The system fails to provide a very compact apparatus, and it lacks simple manufacturing and low manufacturing costs. Additionally, the apparatus is structured for liquid cryogens only, which has its own disadvantages, as discussed. This leads to a lack of maximum heat transfer/high efficiency coefficient.
As such, gaseous cryogens may be preferred to overcome the drawbacks of liquid cryogens. However, using only gaseous cryogens makes the cable more sensitive to heat influx since the heat capacity of gas cryogens typically is significantly inferior to or lower than the heat capacity of liquid cryogens. In other words, gaseous cryogens, by themselves, are insufficient to maintain a proper superconducting environment at and close to the terminations.
Another method of cooling high temperature superconducting material is discussed in U.S. Pat. No. 7,748,102 B2 to Manousiouthakis et al. (“Manousiouthakis”). Manousiouthakis provides an apparatus where HTS wire is surrounded by an inner layer of thermal insulator, a layer of high thermal conductivity material such as copper, and an outer layer of thermal insulator with cryogenic coolant sources distributed along the power transmission cable and coupled to the copper layers and HTS wire. The result is a compact apparatus for cooling HTS devices. However, this apparatus also lacks a maximum heat transfer/high efficiency coefficient.
Accordingly, what is needed is an effective apparatus and method for reducing or maintaining temperatures of HTS devices using gaseous cryogens. Additionally, there exists a need for a compact, vacuum tight apparatus for cooling HTS devices with maximum heat transfer/high efficiency coefficient. This can be achieved through implementation of a heat sink to intercept the heat leak from the room temperature components to the superconducting cable. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.
All referenced publications are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.